In 1926, US 141 ran from Green Bay to Milwaukee. The photo below shows the
original north terminus in Green Bay:

Enslin, 2001

Here's what Jon had to say:

"This is the original northern terminus of US 141 in
Green Bay WI, at the corner of Bodart and Monroe (Monroe is the
cross street). At the time, Bodart Street was Main Street and ran
straight through this intersection (basically original Main ran
right through the building). Over the course of time, Main was
moved one block north, yet still curves back to cross the Fox
River behind this building. Originally US 41 came in from the
left, and then straight ahead along Main. US 141 was extended
along Main with US 41 in 1929."

In 1929 the north end of US 141 was extended to a point northeast of Covington MI; you can view photos from there on this
page.

Today the south end of US 141 is not far from its original northern terminus: just outside Green Bay WI, at its
interchange with I-43 near a community called Bellevue. But US 141
originally continued south from there, roughly along the path of
I-43, through Manitowoc and Sheboygan, and ended at US 18 in
Milwaukee:

Enslin, 2001

Jon writes: "This is what I believe was the original
southern terminus of US 141 in Milwaukee, at the corner of 20th
Street and Highland Avenue. US 18 still exists, as it always has,
on Highland. US 41 used to approach this intersection from
straight ahead on Highland, and then off to the right on 20th. WI
hwy. 57 currently runs through this intersection. At that time US
16, WI 19, and WI 30 did as well. That means six highways were
involved with this intersection at one point."

I've enlarged the two sign assemblies below:

Enslin, 2001

Enslin, 2001

"This is another southern terminus of US 141 in
Milwaukee, at the corner of 27th Street and Highland (7 blocks
west of the previous picture). In the 1930's, the routing of US 41
was slightly altered to run along 27th. I have a map that clearly
shows US 141 ending here. US 18 still exists, as it always has, on
Highland at this point."

Enslin, 2001

"Yet another southern terminus of US 141 in Milwaukee.
This is the corner of Port Washington Road and Capitol Drive. US
141 ended here when the current I-43 freeway was under
construction in Milwaukee County. Although Capitol Drive is a
former routing of US 16, WI 190 ran along Capitol as it does now -
meaning at one point in time US 141 ended at a state highway."

Morrison, May 2010

"This is the final southern terminus of US 141 in
Milwaukee. This is the Marquette Interchange downtown. I-43 replaced US 141
at this point in 1977."

In 1977, US 141 was truncated shy of Milwaukee. By 1981, it had been truncated
incrementally three more times, until finally the south terminus was at its
current location in Bellevue. The photo below is looking north on I-43:

me, Aug. 2005 (and I'm told all of these are unchanged as of Dec. 2010)

At the end of that offramp, the assembly below is posted:

me, Aug. 2005

Approaching from the south on County Hwy. MM, the beginning of US 141 is shown
thus:

me,
Aug. 2005

Not far ahead from there, you reach the junction shown below:

me,
Aug. 2005

Today's County Hwy. R used to be southbound US 141, back when the route continued
to Manitowoc and beyond. Now the designation only goes left; if you take that
turn, you'll soon see the first reassurance marker:

me,
Aug. 2005

Heading southbound on US 141, the "End" assembly shown below is posted
after making the turn off from historic US 141 and onto the road that has an
interchange with I-43: